nir/builder: Add an init function that creates a simple shader for you

nir/builder: Add an init function that creates a simple shader for you

A hugely common case when using nir_builder is to have a shader with a
single function called main.  This adds a helper that gives you just that.
This commit also makes us use it in the NIR control-flow unit tests as well
as tgsi_to_nir and prog_to_nir.

Reviewed-by: Eric Anholt <eric@anholt.net>
Reviewed-by: Connor Abbott <cwabbott0@gmail.com>
Acked-by: Kenneth Graunke <kenneth@whitecape.org>